View Single Post
Old 22nd Mar 2018, 7:25 am   #11
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
Default Re: Armstrong 625 receiver

The AM tuner is very unusual.

The ferrite rod is untuned and feeds a broadband buffer amp. The external antenna connector bypasses this.

A fixed lowpass filter feeds a single-balanced diode mixer which converts the wanted signal to about 3.1 MHz where it gets filtered by two IFTs with an AGC'd amplifier between them.

Next comes a dual gate mixer taking the signal down to 455kHz and a ceramic filter. The second LO is a free-rinning LC job.

The first LO is tuned by a single varactor with voltage from a pot or a bank of preset pots.

It ought to sound OK, but if the second LO is off frequency, then the ceramic filter and the 3.1MHz selectivity could fail to coincide, or there could be something wrong with the ceramic filter.

Designed with 9kHz channel spacing in mind and with a tight-ish ceramic filter, it's not going to give the audio bandwidth of classic AM sets, but 1kHz drop off sounds like there's a fault.

A mini-communications receiver on the broadcast band! The front-end is wide open to overload from any strong signals.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now